Distribution of Fishes in the Vicinity of Dredging Operations in the James River,Virginia

摘要

Potential effects of dredging-induced underwater noise,suspended sediment plumes,and hydraulic entrainment have been identified as sources of concern for protection of fishery resources.Although such concerns have persisted for decades,conclusive evidence either substantiating or refuting them is lacking. One means of establishing the likelihood of actual impacts to fishery resources is examining the distribution of fishes in relation to the conduct of the dredging project. In the present study,fisheries hydroacoustic techniques were used to examine abundances and spatial and temporal distribution patterns of fishes around a hydraulic cutterhead dredging operation in a tidal reach of the James River,Virginia in October 1999.Hydroacoustic surveys were conducted during various stages of the tide on two spatial scales:several kilometers up and downstream of the dredge,and within several hundred meters of the dredge site or pipeline discharge area.Surveys were completed during periods of active dredging as well as when the dredge was idle. Gill nets were deployed to characterize the local fish assemblage and to identify acoustic targets.White perch(Morone americana),spot(Leiostomus xanthurus),and gizzard shad(Dorosoma cepedianum)accounted for almost half of all fish captures. Estimates of fish density with distance(30 m increments)from the dredge were examined for evidence of fish attraction to,or avoidance of,an operating dredge. Synoptic ADCP data were used to characterize flow fields and acoustic backscatter along the same survey lines.Detectable signatures of dredging-induced suspended sediment plumes against highly variable ambient conditions were limited to within 100-200 m of the dredge or discharge area.Neither lower(avoidance response)nor higher(attraction response)than expected densities were observed in relation to the location of the dredge or the discharge plume.Average densities of fish targets were nearly equivalent for wide area surveys for an active（3.3 fish/100m 3 )and idle dredge （3.6 fish/100m 3 ).Fish distribution patterns appeared to be influenced primarily by water depth,as fish targets predominantly occurred off the bottom in deeper waters of the natural and dredged channel sections.This finding was consistent with the gill net catch data.Distributions did not change substantially across tidal stages.Mid- water（3.3-5.7 m)fishes had consistently lowest overall echo integrated fish density estimates,ranging from 1.8 to 6.1 fish/100m 3 (mean=3.3 fish/100m3 ).Overall densities of surface(mean=4.8fish/100m3)and bottom(mean=5.0 fish/100m3) fishes were comparable.Densities observed in the dredged material placement area did not change during active discharges as compared to non-pumping periods.The presence of an active dredging operation did not shift fish distributions in a detectable manner.Further studies are planned to examine species-specific responses.